Updated the Symfony Installer installation instructions
bug #4494 Updated the Symfony Installer installation instructions (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #4494).

Discussion
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Updated the Symfony Installer installation instructions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

@weaverryan @WouterJ @xabbuh I'm truly sorry for *pushing* you, but this is a **ultra-high priority PR** because the current Symfony installation instructions displayed on http://symfony.com/doc/current/quick_tour/the_big_picture.html are broken.

Commits
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f374bf4 Updated the Symfony Installer installation instructions